Enabling gobject-introspection support in gtk3 builds

Hi,

Part of Pranav's GSoC project was to tweak the gtk widget in 
libreofficekit/ so that it follows the rules that allows access to it 
from other languages. For example, gnome-document's use-case is 
javascript.

The commit to enable that for distributions is at
<https://gerrit.libreoffice.org/#/c/18494/>. configure is updated to
check for the newly required necessary tools. My understanding is that
we prefer build-time tools to be required by default and to be able to
switch it off with a configure switch, rather than auto-disable features
magically.

If that's the case, it can happen that some tinderbox that has gtk3
enabled, but lacks the gobject-introspection tools will fail early in
configure with the patch in, and will need action from the tinderbox
owner.

Knowing that, am I OK to push the patch to master?

(The patch does not affect OS X / Windows builds, but in case Jenkins
provides feedback in a reasonable amount of time, I'll of course not
ignore it.)

Thanks,

Miklos

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